Oxidation of Lignin
The oxidation of lignin was performed in a 50 mL autoclave (Labe Scientific Instrument Co., Ltd., Shanghai, China) with Teflon inner. In a typical reaction, 0.1 g lignin, desired amount of catalyst and 10 g pre-prepared microemulsion mixture were added in the autoclave, then 1.0 MPa O2 was used to purge it three times and pressurized. Subsequently, the autoclave was heated and kept at a designated temperature for 1-5 h under the stirring rate of 600 rpm. After the reaction, the autoclave was cooled with flowing water to room temperature. The reaction mixture was diluted by ethanol to a fixed volume of 25 mL, among which 5 mL was fetched and dried by anhydrous sodium sulfate for GC-MS analysis. Then, 50 mL deionized water was added into other 20 mL solution for reprecipitating lignin. So recovered lignin (Re-lignin) was obtained by the filtration and then washed by water and dried for 4 h at 333 K for subsequent characterization.
Measurement and Characterization
The subregions of microemulsions were distinguished through electrical conductivity measurements (DDSJ-308A, DSJ-0.1C electrode, Shanghai Precision Scientific Instrument Co., Ltd). During analysis, suitable amount of acetic acid was added into the oil phase, with the concentration about 50 mmol L-1.27 The micro-polarities of different microemulsions using methyl orange (MO) as the probe and the solubility of lignin were measured by UV-vis spectroscopy (UV-2450, Shimadzu, Japan) at room temperature with 0.1 nm resolution. Lignin concentrations in different microemulsions were calculated by an external standard method, where every test was carried out in triplicate. Cryogenic scanning electron microscopy (Cryo-SEM) was performed on a JSM-7100F SEM (JEOL Ltd, Tokyo, Japan) at 133 K. Before analysis, the samples were pre-cooled by liquid nitrogen and stayed in the preparation chamber (PP3010T Cryo-SEM Preparation System, Quorum Technologies, UK) at 133 K. The formation of different microemulsions, and the structures of raw lignins and Re-lignin were characterized by FT-IR spectrophotometer (Bruker Tensor 27) by the KBr pellets with 4 cm-1 resolution and 64 scan times.
Volatile products were identified by GC-MS (Agilent 7890B/5977A, HP-INNOWAX capillary column: 30 m × 0.32 mm × 0.25 μm) on the base of Agilent MS library. The temperatures of the injection and detector were fixed to 553 K. The oven temperature was started at 323 K (held for 1 min), then programmed to 533 K at the rate of 10 K min-1 and finally kept at 533 K for 15 min. The quantitative analysis was performed by GC (Agilent 7890B) equipped with a FID detector using the same chromatography column and temperature program as above GC-MS analysis. The yields of phenolic monomers were measured by using dimethyl phthalate as the internal standard and calculated through following equations:
Y = \(\frac{A_{i}\text{\ M\ W}}{\text{A\ }M_{L}}\ \)x 1000 mg g-1 (Equation 1)
Y = mg g-1 (Equation 2)
where Yi and Ai are the yield and peak area of the product i , Y is the total yield of phenolic monomers. M and ML represent the mass of internal standard solution and lignin (g), W andA represent the mass concentration and peak area of internal standard, respectively.